 
 (def rand-density-unit (rand-unit :density))
 (def rand-energy-unit (rand-unit :energy))
+(def rand-force-unit (rand-unit :force))
 (def rand-length-unit (rand-unit :length))
 (def rand-mass-unit (rand-unit :mass))
 (def rand-pressure-unit (rand-unit :pressure))

                   [:kg-per-m3 :lb-per-cu-ft] #(/ % (factors [:lb-per-cu-ft :kg-per-m3]))
                   [:ft-lb :J]                #(* % (factors [:ft-lb :J]))
                   [:J :ft-lb]                #(/ % (factors [:ft-lb :J]))
                   [:ft :m]                   #(/ % (factors [:m :ft]))
                   [:m :ft]                   #(* % (factors [:m :ft]))
                   [:lb :kg]                  #(/ % (factors [:kg :lb]))

(defn rand-unit [physical-quantity]
  (let [quantity-units-map {:density                [:lb-per-cu-ft :kg-per-m3]
                            :energy                 [:ft-lb :J]
                            :force                  [:lbf :N]
                            :length                 [:ft :m]
                            :mass                   [:lb :kg]
                            :pressure               [:psi :kPa]
